1. Field of the Invention
The present invention relates to an information recording-reproducing method comprising recording and reproducing information on and from a recording medium, and particularly to an information reproducing method utilizing domain wall displacement. The present invention also relates to a magneto-optical medium used in this method.
2. Related Background Art
In recent years, great expectation has been entertained of magneto-optical recording-reproducing apparatus using a magneto-optical disk as a recording medium in that they are portable, have a great memory capacity and are erasable and rewritable. FIG. 1 illustrates an optical head for such a magneto-optical recording-reproducing apparatus. In FIG. 1, reference numeral 15 designates a semiconductor laser as a light source. A divergent flux of rays emitted from the semiconductor laser 15 is made parallel by a collimator lens 16 and then rectified to a parallel flux of rays of a circular form in section by a beam-shaping prism. In this case, linearly polarized light components perpendicular to each other are regarded as P polarized light and S polarized light, and this parallel flux of rays is regarded as linearly polarized light of P polarized light (here, linearly polarized light in a direction parallel to the drawing).
The light flux of P polarized light is incident on a polarized light beam splitter 18. The polarized light beam splitter 18 is characterized by, for example, a transmittance of 60% and a reflectance of 40% for the P polarized light, and a transmittance of 0% and a reflectance of 100% for the S polarized light. The light flux of the P polarized light passed through the polarized light beam splitter 18 is focused by an objective lens 19 and is projected as a light spot on a magnetic layer of a magneto-optical disk 20. An external magnetic field is applied from a magnetic head 21 to this light-spot projected portion, so that a magnetic domain (mark) is recorded on the magnetic layer by the irradiation of the light spot and the application of the external magnetic field.
Reflected light from the magneto-optical disk 20 is returned to the polarized light beam splitter 18 through the objective lens 19. A part of the reflected light is separated here and afforded to a reproducing optical system. In the reproducing optical system, the light flux separated is further separated by a polarized light beam splitter 22 separately provided. The polarized light beam splitter 22 is characterized by, for example, a transmittance of 20% and a reflectance of 80% for the P polarized light, and a transmittance of 0% and a reflectance of 100% for the S polarized light. One light flux separated by the polarized light beam splitter 22 is guided to a half prism 29 through a condenser lens 28. The light flux is divided into two portions here, and one is guided to a photosensor 30, and the other to a photosensor 32 through a knife edge 31. Error signals for auto-tracking and auto-focusing of a light spot are generated by these controlling optical systems.
The other light flux separated by the polarized light beam splitter 22 is guided to a half-wave plate 23 for turning the polarizing direction of the light flux by 45 degrees, a condenser lens 24 for focusing the light flux, a polarized light beam splitter 25, and photosensors 26 and 27 for separately detecting light flux portions separated by the polarized light beam splitter 25 to reproduce information. The polarized light beam splitter 25 is characterized by a transmittance of 100% and a reflectance of 0% for the P polarized light, and a transmittance of 0% and a reflectance of 100% for the S polarized light. Signals detected by the photosensors 26 and 27 are differentially detected by a differential amplifier (not illustrated), thereby generating a reproduction signal.
In a magneto-optical medium, information is recorded by a difference in the direction of vertical magnetization. When the magneto-optical medium, in which information has been recorded by the difference in the direction of magnetization, is irradiated with linearly polarized light, the polarizing direction of light reflected therefrom is turned either clockwise or counterclockwise according to the difference in the direction of magnetization. For example, when a polarizing direction of linearly polarized light incident on the magneto-optical medium, and directions of reflected light for downward magnetization and reflected light for upward magnetization are regarded as directions of a coordinate axis P, R+ turned by +.theta.k and R- turned by -.theta.k, respectively, as illustrated in FIG. 2, and an analyzer is arranged in such a direction as illustrated in FIG. 2, light passed through the analyzer becomes A for R+ or B for -R. When this light is detected by a photosensor, information can be obtained as a difference in intensity of light. In the example illustrated in FIG. 1, the polarized light beam splitter 25 plays a part of the analyzer and serves as an analyzer in a direction turned by +45 degrees from the axis P for one light flux separated or in a direction turned by -45 degrees from the axis P for the other light flux. Namely, the signal components obtained by the photosensors 26 and 27 become antiphase. Therefore, the individual signals are differentially detected, whereby a reproduction signal can be obtained with reduced noise. In FIG. 2, axis S means an axis for S polarized light direction. Points S+ and S- on axis S mean S coordinates of points R+ and R- when letting the coordinates of points R+ and R- be (P+, S+) and (P+, S-) in P-S coordination system, respectively.
Recently, there has been a strong demand for enhancing the recording density of this magneto-optical medium. In general, the recording density of an optical disk such as a magneto-optical medium depends on the laser wavelength and the NA (numerical aperture) of an objective lens of a reproducing optical system. More specifically, since the laser wavelength .lambda. and the NA of the objective lens of the reproducing optical system decide the diameter of a light spot, the range of a reproducible magnetic domain is limited to about .lambda./2NA. Therefore, for actually achieving higher recording density with a conventional optical disk, it has been necessary to shorten the laser wavelength or enlarge the NA of the objective lens in the reproducing optical system. However, the improvements in the laser wavelength and the NA of the objective lens are limited naturally. Therefore, techniques that the construction and reading method of a recording medium are devises to improve the recording density have been developed.
For example, in Japanese Patent Application Laid-Open No. 6-290496, there is proposed a domain wall displacement-reproduction system in which a reproduction signal is obtained after a light spot is scanned on a track on a magneto-optical medium composed of a laminate of plural magnetic layers, thereby transferring a magnetic domain (mark) recorded as vertical magnetization on a first magnetic layer to a third magnetic layer arranged with interposition of a second magnetic layer for adjusting exchange-coupling force, and a domain wall of the magnetic domain transferred to the third magnetic layer is displaced, thereby making the magnetic domain larger than the magnetic domain (mark) recorded on the first magnetic layer.
FIGS. 3A to 3D illustrate the principle of the domain wall displacement-reproduction method. FIG. 3A is a cross-sectional view of magnetic layers of a magneto-optical medium, and FIG. 3B is a plan view viewed from the side on which a light spot is incident. In FIG. 3A, an arrow A indicates a moving direction of the medium. Reference numeral 33 designates a magneto-optical disk as the magneto-optical medium. Reference numeral 34 indicates a first magnetic layer which is a memory layer for recording information as a magnetic domain (mark). Reference numeral 35 indicates a second magnetic layer which is an switching layer for adjusting exchange-coupling force between the memory layer 34 and a third magnetic layer 36. The third magnetic layer 36 is a displacement layer to which the magnetic domain recorded on the memory layer 34 is transferred, and which makes the transferred magnetic domain larger than the magnetic domain recorded on the memory layer 34 by displacing a domain wall of the transferred magnetic domain using the function of the switching layer 35 and the heat distribution by the light spot. In FIG. 3A, an arrow B indicates a moving direction of the domain wall.
These magnetic layers are exchange-coupled to one another at room temperature. At a temperature close to ambient temperature, the first magnetic layer 34 is relatively smaller in domain wall coercive force and greater in domain wall displaceability compared with the third magnetic layer 36. The second magnetic layer 35 is composed of a magnetic film having a Curie temperature lower than the first and third magnetic layers. Reference numeral 37 indicates a light spot for reproduction, and 38 a track intended to reproduce on the magneto-optical disk 33. Arrows in the respective layers of the memory layer 34, the switching layer 35 and the displacement layer 36 indicate directions of atomic spins. Such a domain wall as indicated by 39 is formed between regions in which the directions of spins are opposite to each other. Reference numeral 40 indicates a domain wall, which is being displaced, within the magnetic domain transferred to the displacement layer 36.
FIG. 3C diagrammatically illustrates a temperature distribution formed on the magneto-optical disk 33, in which an axis of ordinate and an axis of abscissa indicate a medium temperature T and a position X, respectively. Domain wall displacement-reproduction may be theoretically feasible by using either one light spot or two light spots. For the sake of brevity of description, reproduction using two light spots will be described here. Only a light spot contributing to a reproduction signal is illustrated in FIG. 3B. The second light spot (not illustrated) is projected for forming the temperature distribution illustrated in FIG. 3C. Now, suppose that the temperature on the magneto-optical disk 33 at a position Xs is a temperature Ts close to the Curie temperature of the switching layer 35, a hatching portion 41 in FIG. 3A indicates a region in which the temperature is above the Curie temperature.
FIG. 3D diagrammatically illustrates the distribution of domain wall energy density .sigma.1 of the displacement layer 36 corresponding to the temperature distribution in FIG. 3C, in which a left axis of ordinate and a right axis of ordinate indicate domain wall energy density .sigma. and force F acting on the domain wall, respectively. When a gradient of the domain wall energy density .sigma. exists in a direction of X as illustrated in FIG. 3D, the force F1 illustrated in FIG. 3D acts on the domain walls of the respective magnetic layers existing at the position X so as to displace the domain walls to a position at which the domain wall energy is lower. Since the displacement layer 36 is small in domain wall coercive force and great in domain wall displaceability, its domain wall is easily displaced in itself by this force F1. In a region on this side of Xs (right-hand side of the center in FIGS. 3A and 3B), however, the temperature of the magneto-optical disk 33 is yet lower than Ts, and so the domain wall in the displacement layer 36 also comes to be fixed at a position corresponding to the position of the domain wall of the memory layer 34 by the exchange-coupling with the memory layer 34 great in domain wall coercive force.
Now, when the domain wall 40 is at a position Xs as illustrated in FIG. 3A, and the temperature of the magneto-optical disk 33 at the position Xs is raised to Ts close to the Curie temperature of the switching layer 35, and so the exchange-coupling between the displacement layer 36 and the memory layer 34 is broken, the domain wall 40 in the displacement layer 36 is momentarily displaced to a region in which the temperature is higher and the domain wall energy density is lower, as shown by the arrow B. Accordingly, when the light spot 37 for reproduction passes through the region, the atomic spins of the displacement layer 36 within the light spot are all aligned in one direction as illustrated in FIG. 3B. With the movement of the medium, the domain wall 40 (or 39, or the like) is momentarily displaced, and the directions of atomic spins within the light spot are reversed to align in one direction.
With respect to the reflected light from the magneto-optical disk 33, a reproduction signal is detected by the same differential detection as in the conventional optical head illustrated in FIG. 1. However, the signal reproduced by the light spot always become a fixed amplitude irrespective of the size of magnetic domains recorded on the memory layer 34, which eliminates the problem of waveform interference caused by optical diffraction limit. More specifically, the use of the domain wall displacement-reproduction permits reproduction of magnetic domains (marks) smaller than the resolving power limit determined by the laser wavelength .lambda. and the NA of the objective lens, i.e., about .lambda./2NA, and reproduction at a submicron linear density.
FIG. 4 illustrates an example of the construction of an optical head in the case where 2 light spots are used. In FIG. 4, reference numeral 42 designates a semiconductor laser for record reproduction, the wavelength of which is 780 nm by way of example. Reference numeral 43 indicates a semiconductor laser for heating, the wavelength of which is 1.3 .mu.m by way of example. Both lasers are arranged in such a manner that rays emitted therefrom are incident in the form of P polarized light on a recording medium. Laser beams emitted from the semiconductor lasers 42 and 43 are shaped in a substantially circular form by beam-shaping means (not illustrated) and then rectified to parallel fluxes of rays by respective collimator lenses 44 and 45. Reference numeral 46 designates a dichroic mirror designed to completely transmit light of 780 nm and completely reflect light of 1.3 .mu.m. Reference numeral 47 indicates a polarized light beam splitter 18 designed to transmit 70 to 80% of P polarized light and almost completely reflect S polarized light which is a component perpendicular to the P polarized light.
The light fluxes made parallel by the collimator lenses 44 and 45 are incident on an objective lens 48 through the dichroic mirror 46 and the polarized light beam splitter 47. At this time, the light flux of 780 nm is made larger than the aperture size of the objective lens 48, while the light flux of 1.3 .mu.m is made smaller than the aperture size of the objective lens 48. Accordingly, the NA of the lens acts smaller on the light flux of 1.3 .mu.m though the same objective lens 48 is used, whereby the size of a light spot on a recording medium 49 becomes larger than that of the light flux of 780 nm. Reflected light from the recording medium passes through the objective lens 48 again into a parallel flux of rays. This light flux is reflected by the polarized light beam splitter 47 into a light flux 50. After the light flux 50 is subjected to wavelength separation and the like in another optical system (not illustrated), a servo error signal and an information-reproduction signal are obtained in the same manner as in the conventional system.
FIGS. 5A and 5B illustrate a relationship between a light spot for record reproduction and a light spot for heating on a recording medium. In FIG. 5A, reference numerals 51 and 52 indicate a light spot of wavelength of 780 nm for record reproduction and a light spot of wavelength of 1.3 .mu.m for heating, respectively. Reference numeral 53 designates a domain wall of a magnetic domain recorded on a land 54, and 55 a groove. Reference numeral 56 indicates a region in which the temperature T has been raised by the light spot for heating. Such a temperature gradient as illustrated in FIG. 5B can be formed on a moving recording medium by combining the light spot 51 for record reproduction and the light spot 52 for heating on the land 54 between the grooves 55. A relationship between the temperature gradient and the light spot for record reproduction becomes the same as that illustrated in FIGS. 3B and 3C, whereby the domain wall displacement-reproduction can be conducted.
The above description is about the domain wall displacement-reproduction system using two beams. However, reproduction by one beam is desirable in view of the simplification of apparatus. Operation in the case where domain wall displacement-reproduction is conducted by one beam is described with reference to FIGS. 6A and 6B. FIG. 6A is a cross-sectional view of a magneto-optical disk 33 like FIG. 3A. The disk 33 is composed of a memory layer 34, an switching layer 35 and a displacement layer 36. FIG. 6B is a plan view viewed from the side on which a light spot is incident. Reference numerals 54 and 55 indicate a land of a track and a groove, respectively. Reference numeral 57 indicates a light spot for reproduction. When the light spot 57 is projected, a temperature distribution indicated by an oval contour as illustrated in FIG. 6B is formed on the recording medium. The moving direction of the medium is a direction of an arrow A. Arrows in the respective layers of the magneto-optical medium 33 indicate directions of atomic spins.
A region indicated by 60 is a high-temperature portion heated to a temperature above the Curie temperature of the switching layer 35, and so the magnetization of the switching layer 35 disappears. Therefore, the exchange-coupling between the memory layer 34 and the displacement layer 36 is broken in the region of the high-temperature portion 60, so that a magnetic domain of the memory layer 34 is not transferred to the displacement layer 36. In other regions than the high-temperature portion 60, the magnetic domain of the memory layer 34 is transferred to the displacement layer 36, since exchange-coupling power functions. When domain walls 58 and 59 of the magnetic domain recorded on the memory layer 34 come to respective boundaries between a low-temperature region and the high-temperature region 60, the domain walls 58 and 59 are displaced toward the high-temperature portion in directions of an arrow D and an arrow C, respectively. Reference numeral 61 indicates a region (hatching portion) in which the domain wall 58 is displaced (hereinafter referred to as "front region"), and reference numeral 62 designates a region (hatching portion) in which the domain wall 59 is displaced (hereinafter referred to as "back region"). As apparent from FIGS. 6A and 6B, it is understood that when it is intended to reproduce information by the conventional differential detection as it is, information by the domain wall 58 and information by the domain wall 59 are mixed into the light spot 57, and so the information cannot be correctly reproduced.
The problem offered in the case of one-beam reproduction is described in more detail with reference to FIGS. 7A to 7G. FIGS. 7A to 7F illustrate a state that a light spot 57 for reproduction successively scans a land 54 on a track. A recording medium is moving in a direction of an arrow A like FIG. 6A. Reference numerals 61 and 62 indicate a front region and a back region, respectively. Suppose that an isolated magnetic domain 63 is recorded on the land 54, and for example, the isolated magnetic domain 63 alone is magnetized upward, while the other is magnetized downward. Reference numerals 64 and 65 indicate domain walls on both sides of the isolated magnetic domain 63. FIG. 7G illustrates reproduced waveforms of differential signals obtained at respective positions. First of all, FIG. 7A illustrates a state in a case where the light spot 57 is located at a position distant from the isolated magnetic domain 63.
In this case, both front region 61 and back region 62 are magnetized downward, and a differentially detected signal at this time indicates a ground level as illustrated in FIG. 7G. FIG. 7B illustrates a state in a case where the light spot 57 has come near to the isolated magnetic domain 63. In this case, however, the domain wall 64 does not yet reach the front region 61, and a differentially detected signal indicates a ground level like the case of FIG. 7A. FIG. 7C illustrates a state that the domain wall 64 has just entered the front region 61. In this case, the domain wall 64 of the displacement layer 36 in the front region 61 is displaced toward the high-temperature portion, and a hatching portion indicated by 66 in FIG. 7C becomes a region magnetized upward. A differentially detected signal is changed to a higher level as illustrated in FIG. 7G.
FIG. 7D illustrates a state that the domain wall 65 on the opposite side has just entered the front region 61. In this case, the domain wall 65 of the displacement layer 36 in the front region 61 is displaced toward the high-temperature portion and returned to downward magnetization. A differentially detected signal is also returned to the ground level as illustrated in FIG. 7G. FIG. 7E illustrates a state in a case where the light spot 57 is further advanced, and the domain wall 64 has just entered the back region 62. In this case, the domain wall 64 of the displacement layer 36 in the back region 62 is displaced toward the high-temperature portion, and a hatching portion indicated by 67 becomes a region magnetized upward. A differentially detected signal is changed to a medium level as illustrated in FIG. 7G. The reason why the signal level becomes lower than the case of the front region 61 is that the center of the high-temperature portion is situated on the back side of the center of the light spot 57. FIG. 7F illustrates a state that the domain wall 65 on the opposite side has just entered the back region 62. In this case, the domain wall 65 of the displacement layer 36 in the back region 62 is displaced toward the high-temperature portion and returned to downward magnetization. A differentially detected signal is also returned to the ground level.
As described above, the domain wall is displaced at two places of the front region and the back region for one isolated magnetic domain when the domain wall displacement-reproduction system using one beam is used, so that two pulses are obtained naturally. Since magnetic domains are optionally recorded for actual signals, the contribution of the domain wall displacement in the front and back regions to a differentially detected signal cannot be distinguished because of their complicated mixing. As a method for solving this problem, there is a method using a Curie temperature mask of the displacement layer 36 as illustrated in FIGS. 8A and 8B. FIG. 8A is a cross-sectional view of a magneto-optical disk 33 like FIG. 6A, and FIG. 8B is a plan view viewed from the side on which a light spot is incident. Reference numeral 68 indicates a light spot for reproduction. As with the case illustrated in FIG. 6B, a temperature distribution indicated by an oval contour is formed on the magneto-optical disk 33.
In this method, the power of the light spot 68 is preset higher than the case illustrated in FIGS. 6A and 6B. A hatching region indicated by 69 is a high-temperature portion heated to a temperature above the Curie temperature of the displacement layer 36, and so the magnetization of the displacement layer 36 in the region of the high-temperature portion 69 disappears, and the domain wall is also not displaced. Hatching regions indicated by 70 on both sides of the high-temperature portion 69 are medium-temperature portions heated to a temperature above the Curie temperature of the switching layer 35, but lower than the temperature of the high-temperature portion 69.
As described in FIGS. 6A and 6B, the exchange-coupling between the memory layer 34 and the displacement layer 36 is broken in the regions of the high-temperature portion 69 and the medium-temperature portion 70, and so the magnetic domain of the memory layer 34 is not transferred to the displacement layer 36. On the other hand, at low-temperature portions on the outsides of the medium-temperature portions 70, the magnetic domain of the memory layer 34 is transferred to the displacement layer 36, since exchange-coupling power functions. When domain walls 73 and 74 of the magnetic domain recorded on the memory layer 34 come to respective boundaries between the low-temperature region and the medium-temperature portion 70, the domain walls 73 and 74 are displaced toward the high-temperature portion in directions of an arrow B and an arrow C, respectively. Reference numeral 71 indicates a front region in which the domain wall 73 is displaced, and reference numeral 72 designates a back region in which the domain wall 74 is displaced. As apparent from FIG. 6B, it is understood that the back region 72 is beyond the light spot 68 due to the high-temperature portion 69 heated to a temperature exceeding the Curie temperature of the displacement layer 36. In such a manner, the reproduction signal of information by differential detection is prevented from being affected by the back region 72, so that the domain wall displacement-reproduction by one beam can be successfully conducted.
As described above, domain wall displacement-reproduction includes two methods of reproduction by one beam and reproduction by two beams. The reproduction method by two beams, in which another light spot for reproduction than a light spot for heating is projected on a medium, requires two reproducing optical systems, so that the constitution of the apparatus is complicated.
On the contrary, the reproduction method by one beam can simplifies the constitution of the apparatus. Since a domain wall displaced from the front side of the light spot for reproduction toward the high-temperature portion within the light spot for reproduction, and a domain wall displaced from the back side of the light spot for reproduction toward the high-temperature portion within the light spot for reproduction are detected at the same time, however, the information cannot be correctly reproduced. For solving this problem, a method using a Curie temperature mask as has been described by FIGS. 8A and 8B is considered. This method permits the domain wall displacement-reproduction by one beam because the signal from the back region can be inhibited. However, this method has involved a problem that a signal to noise ratio is lowered because the front region also becomes small.